The present invention relates to an image reading apparatus, and an image reading method.
In general, in an image reading apparatus, an operating environment is adjusted beforehand lest the density value of image data to be outputted should exceed a predetermined range. Concretely, the lowest density value is inspected as to white reference digital data which is acquired by reading the optical image (white reference image) of a white reference plate or the like, and the operating environment is adjusted so that the lowest density value may become a density value within the predetermined range. Incidentally, the “lowest density value” shall signify a value which expresses “being the lightest”, among density values expressive of light and shade. The operating environment to be adjusted is set depending upon the luminous intensity of a light source for illuminating an original, the accumulation time of the signal charges of photosensor elements in an image sensor, the gain of groups of analog data outputted from the image sensor, and so forth. It is assumed by way of example that the predetermined range of density values be 30–240 in digital data of 256 gradations from 0 to 255 as are outputted from an A/D converter of 8-bit resolution. On this occasion, in a case where the white reference digital data is inspected by reading the white reference plate and where any pixel whose density value exceeds 240 exists, the operating environment for the image sensor, an analog signal processing unit, etc. is set by adjusting the luminous intensity, an exposure time, the gain of the groups of analog data, etc. so as to lower the density value.
With the enhancement of a resolution, the number of photosensor elements in an image sensor has increased steadily, and a time period which is required for a process for converting groups of analog data outputted as to all the elements, into groups of digital data, and a process for detecting the lowest density value from the groups of the resulting digital data, has lengthened. This bring the problem that a time period which is required for setting an operating environment before the start of the reading of an original image has lengthened.